The present invention relates to a wavelength tunable laser diode and, more particularly, to a wavelength tunable laser diode especially suitable for use with coherent optical communication systems.
Wavelength-division-multiplexing communication involves having a light beam of multiple wavelength carry signals so as to transmit data of multiple channels over a single optical fiber cable. Thus an increased transmission capacity is made available with this method. In this case, the receiver side requires installing a laser diode for a local oscillator. The laser diode allows the receiver to tune in through interference to signals of a specific wavelength out of the received wavelength-division-multiplexed light. Generally, a wavelength tunable laser diode is used as the laser diode for the local oscillator. The wavelength tunable laser diode permits selection of signals of any specific wavelength out of a plurality of wavelengths in the received light. This laser diode must meet three requirements: (1) the tunable range must be wide; (2) the spectral line width must be narrow enough to permit utilization of interference; and (3) the optical output power must be high so as to improve the S/N ratio for the signal.
A first example of the prior art wavelength tunable laser diode is one described in "Electronics Letters," Vol. 25, No. 15 (1989), pp. 990-991. With a grating arrangement inside, this laser diode is a distributed feedback laser diode having a device length of 1.2 mm. The large device length allows the electrode arrangement to be divided into a plurality of electrodes. Varying the injecting current ratio between the multiple electrodes provides a laser beam having a tunable range of 2.2 nm, an optical output power of 20 mW or more and a spectral line width of 1 MHz or less.
A second example of the prior art wavelength tunable laser diode is one described in "Electronics Letters," Vol. 26, No. 1 (1990), pp. 46-47. This is a distributed feedback laser diode with a waveguide inserted therein, the waveguide varying the refractive index. The laser diode of this structure provides a tunable range of 7 nm while its optical output is as low as 4 mW or less.
A third example of the prior art wavelength tunable laser diode is one that utilizes one known laser diode characteristic: that the wavelength typically varies at a rate of 0.1 nm/.degree.C. The laser diode of the above characteristic is attached to a Peltier device used for temperature control. As the temperature of the laser diode is varied, the wavelength emitted therefrom is controlled accordingly.
With the first prior art example comprising multiple electrodes, the reflecting band width of 3 nm for the internal grating limits the tunable range to about 3 nm. Given that constraint, it is difficult to widen the tunable range to 4 nm or more.
With the second prior art example incorporating the refractive-index-tuning waveguide, the carrier doped to vary the refractive index worsens the optical output power through absorption. Given that kind of laser diode, it is impossible to obtain an optical output of 10 mW or higher.
With the third prior art example that uses the Peltier device for laser diode temperature control, the response time of temperature change is prolonged. That is, wavelengths are switched in units of as long as seconds. This is because the Peltier device is attached to the back of the substrate detached from an active layer of the laser diode, with the result that the temperature of the whole laser diode can only be controlled from the back of the substrate.
As described, the conventional wavelength tunable laser diodes have yet to fulfill in practical terms the requirements of output, tunable range and response time at the same time. There has been an urgent need to develope a laser diode that would meet all such requirements.